Physical vapor deposition (PVD) is a process wherein metal and other materials are vaporized and caused to condense on a workpiece surface. The process is adapted for production and is currently used to deposit oxidation and corrosion resistant coatings, such as MCrAlY, on superalloy gas turbine airfoils which are used at high temperatures. The process produces an as-deposited coating which has a columnar grain structure, with the columns running perpendicular to the workpiece surface. The lack of continuity between the adjacent columns, and the presence of bigger vertical defects, called leaders, can affect the performance of the coatings. Therefore, it has been common practice to peen coatings and thereafter heat treat them. Typical coatings are of the order of 0.1 mm thick. Relatively low shot peening intensities are usually used. Various shot media can be used: glass beads are most common, with the choice being preferred nominally in the size GB20 (specification SAE-J1173 of the Society of Automotive Engineers), about 0.18-0.30 mm dia.
The peening is intended to mechanically close some of the defects. Further, compressive stresses are imparted to the coating, and in the subsequent heat treatment at high temperature, these are presumed to assist in recrystallization and consolidation of the coating grain boundaries. However, there are problems associated with peening according to the prior art. The inevitable presence of some fractured shot material especially in a mass of impinging glass beads means that fine jagged particles impact the coating. These tend to erode and remove portions of the coating. Additionally, airfoils have very thin trailing edges, as a characteristic feature. Unless these are physically masked, to prevent impingement of glass beads, the coating in these regions will often be chipped. The net result of this limitation is that the coating in the trailing edge region must be used with whatever defects are present in the as-deposited coating. The higher intensity attainable with GB20 glass beads is of the order of 0.45N (Almen strip number, "N" range, in mm, used throughout; see SAE J442). Higher intensities are attainable with material such as S110 steel shot (SAE J827) but damage by chipping of the coating may result.
Thus, within the constraints of avoiding erosion and avoiding chipping elsewhere, only a certain depth of compression and sealing is achieved. Metallographic studies show that in a typical peened 0.1-0.15 mm PVD coating there will still be defects in the half of the coating nearest the substrate surface. That is, the peening is very effective in the portion of the coating nearest the peened surface, but less effective at greater depths within the coating.
Plasma spray coating is another method which is used to physically deposit coatings on workpieces. In this process, molten droplets of metal are impelled toward the surface where they freeze and adhere. While significant improvements have been made in recent years, plasma coatings are still by nature somewhat porous. In contrast to the PVD coatings, the porosity in plasma coatings tends to be more randomly distributed; there is some tendency for the defects to run parallel with the workpiece surface. As in the case of PVD coatings, plasma coated gas turbine airfoils are glass bead peened and heat treated with results and problems similar to those mentioned above. However, plasma coatings further differ from PVD coatings, in that they have a relatively high characteristic roughness. A typical cast and uncoated airfoil for a gas turbine will have a surface finish of about 60 AA (Arithmetic Average expressed in units of 10.sup.-6 inch) with PVD coatings this finish is essentially replicated. But, in plasma coating the surface finish is inferior and of the order of 125-370 AA.
Therefore, in the peening and finishing coatings it is desirable that plasma coatings be given a smoother finish and that the finish of PVD coatings be at least sustained. Experience shows that glass bead peening of coatings does not substantially improve their surface finish. When better finish is sought, mechanical abrasion and other surface removal techniques have been used. These are costly and tend to undesirably remove some of the coating from the component.